Abstract

Many species of animals and plants are supplied with diverse attachment devices, in which morphology depends on the species biology and the particular function in which the attachment device is involved. Many functional solutions have evolved independently in different lineages of animals and plants. Since the diversity of such biological structures is huge, there is a need for their classification. This paper, based on the original and literature data, proposes ordering of biological attachment systems according to several principles: (i) fundamental physical mechanism, according to which the system operates, (ii) biological function of the attachment device, and (iii) duration of the contact. Finally, we show a biomimetic potential of studies on biological attachment devices.

Keywords:

1. Introduction

Attachment devices are functional systems, the purpose of which is either temporary or permanent attachment of an organism to the substrate surface, to another organism, or temporary interconnection of body parts within an organism. Their design varies enormously and is subject to different functional loads (Nachtigall 1974; Gorb 2001). Many species of animals and plants are supplied with diverse attachment devices and their morphology depends on the species biology and particular biological function of the device. There is no doubt that many functional solutions have evolved independently in different lineages (Irschick et al. 1996; Beutel & Gorb 2001; Gorb & Beutel 2001). The evolutionary background and the biology of species influence the specific composition of attachment systems in each particular organism.

The diversity of biological structures is huge and the amount of literature published on the morphology of biological attachment devices is rather big too. By making a comparison of various attachment devices in biology (Nachtigall 1974; Gorb 2001, 2005; Scherge & Gorb 2001), we find out that biological attachment systems can be subdivided into several groups according to the following principles: (i) fundamental physical mechanism, according to which the system operates, (ii) biological function of the attachment device, and (iii) duration of the contact. Eight fundamental attachment mechanisms have been previously recognized: (i) hooks, (ii) lock or snap, (iii) clamp, (iv) spacer or expansion anchor (v) suction, (vi) dry adhesion, (vii) wet adhesion (glue/cement, capillarity), and (viii) friction (Gorb 2001; figure 1). However, various combinations of these principles may also occur in real biological systems. From the biologists' point of view, attachment devices may serve the following functions: (i) attachment of body parts to one another, (ii) attachment during copulation, (iii) phoresy or parasitism, (iv) dynamic attachment during locomotion, and (v) maintenance of position (Gorb 2001; figure 2). According to the time scale of operation, different systems can be subdivided into three main groups: permanent, temporary and transitory ones (Flammang 1996). In this paper, we discuss these classifications of biological attachment devices and draw some conclusions about the general relationships between the attachment mechanism and functional load of a biological attachment system. Finally, we show a biomimetic potential of studies of biological attachment devices.

Different examples of attachment systems according to their biological functions. (a–d) Attachment to substrate, maintenance of position: (a) eggs of the butterfly Pieris brassicae cemented to the cabbage surface; (b) dragline of the spider Lycosa sp. attached to the glass by sticky threads produced by piriform glands; (c) roots of the ivy plant (Hedera helix) adhering to the tree stem bark; (d) plaque of the byssal thread of the mussel Mytilus edulis. (e–g) Attachment devices used in locomotion: (e) tenent hairs of the leg of the fly Calliphora vicina adhering to glass; (f) claws of the spider Oxyopes heterophthalmus adapted for attaching to and walking in the web; (g) smooth adhesive pad of the grasshopper Tettigonia viridissima (freezing fracture). (h–j) Parasitism, predation and phoresy: (h) leg clamp of the lice Pediculus humanus; (i) ventral aspect of the mite Ixodes persulcatus mouthparts used in anchoring in the host tissue; (j) aquatic mites anchoring in the cuticle of an aquatic insect.

2. Biological functions of attachment

Attachment is required to fulfil a number of biological functions. These functions are: (i) maintenance of position, i.e. attachment to the substrate to stay in place, (ii) locomotion requiring strong adhesion and friction with a large number of rapid attachment–detachment cycles, (iii) attachment to the animal or plant host for feeding or phoresy (dispersal), (iv) prey capture and a firm hold of the captured prey, (v) temporary attachment between two body parts, (vi) maintenance of mechanical contact with the mating partner during copulation, and (vii) particle manipulation during grooming, sampling and filtering (figure 2). Below, we provide a short overview of these functions with some selected examples.

Many sessile animals use various physical principles to attach themselves to the substrate to occupy territory. Especially marine organisms, such as mussels (figure 2d), barnacles, sea anemones, sea urchins and many others must withstand rather strong forces generated by waves at the sea shore (Waite 1983, 1988; Flammang 1996; Santos et al. 2005). Sea anemones and sea urchins may stay in place attached for quite a long time; however, they are not sessile organisms in the strict sense because they are capable of movements along the substrate. Also, many climbing plants have developed adhesive structures to secure themselves on the surface of stones and other plants (figure 2c; Scherge & Gorb 2001). In general, one of the many functions of plant roots is anchorage in the soil (Goodman et al. 2001). Spiders use secretion of piriform glands to adhere their safety threads (draglines) to the substrate. By doing so, they secure themselves against occasional falls or ensure a short cut to return to the initial place during hunting or exploratory behaviour (figure 2b; Gorb et al. 1998). Many insects and fishes adhere their eggs to substrates, on which their larvae can potentially feed (Ohta 1984; Patzner & Glechner 1996; Panizzi 2006; figure 2a). Long-term attachment to the substrate usually relies on mechanical interlocking or on the use of cements/glues, but a combination of both principles is observed in most systems.

Although parasitism, predation and phoresy have slightly different requirements on attachment structures, these systems taken together can be still recognized as those providing long-term attachment to an other animal (host or prey). Another case of animal–animal attachment is copulation, which is, due to its very specialized nature and strong sexual dimorphism, considered separately here (see below). Most parasites require an ability to form long-term attachments to their hosts. This function is fulfilled by an action of suction cups (some crustaceans and mites), mechanical interlocking (most cases; figure 2h–j) and by the use of glues or cements. The situation is slightly different in predators, where strong contact forces must be developed very rapidly during prey capture (for review see Betz & Kölsch 2004). Many predatory animals bear specialized surface structures for mechanical interlocking, some of them rely on microstructures enhancing friction (e.g. aquatic bugs: Nepa, Ranatra and Belostoma) whereas some use adhesive hairs (spiders, Foelix 1982; reduviid bugs, Weirauch 2005). In a few cases, suction cups (cephalopod molluscs) or adhesive secretions are involved in capturing prey (Hintzpeter & Bauer 1986; Bauer & Kredler 1988; Betz 1996). However, passive sticky traps are widely spread among orb web spiders (Opell 1999; Hawthorn & Opell 2002, 2003).

Other interesting cases of attachment structures have been previously described at the interfaces between two different parts within the same organism. Some articulations have to be mobile most of the time, but they have to be firmly interconnected in some behavioural situations. For example, covering wings of beetles and bugs are separated from the body in flight, but locked during walking, in order to provide mechanical integrity of the entire body, to stabilize the origin sites of some leg muscles and/or to prevent water loss (Hammond 1989; Samuelson 1996; Gorb 1998a, 1999a; Perez Goodwyn & Gorb 2003; figure 3c). The dragonfly head arrester (figure 3b) mechanically secures the head at two additional points during pairing or feeding (Gorb 1999b) and provides high mobility of the head in flight, when the head is involved in the flight control as a mechanosensory organ (Mittelstaedt 1950). Some animals attach their fore wings to hind wings in flight, in order to be supported by a larger area in flight (functional diptery; Perez Goodwyn & Gorb 2004). Probably the most interesting aspect of the last example is that such an articulation remains immobile in one direction, but provides sliding of the fore wing relatively to the hind wing in the other direction (figure 3a). In birds, keratinous microhooks provide a structural integrity of the feather (Nachtigall 1974; Scherge & Gorb 2001). Principal mechanisms, responsible for additional interconnection of body parts, are mechanical interlocking and friction (Gorb & Popov 2002).

Different examples of attachment systems according to their biological functions. (a–c) Interlocking of body parts: (a) temporary locking mechanism between fore- and hindwings in sawfly (Hymenoptera); (b) two corresponding surfaces covered with microstructures in the head-arresting mechanism in the damselfly Perissolestes romulus; (c) double tongue-and-groove joint between the right and left covering wings in the beetle Tenebrio molitor. (d–f) Attachment devices used during copulation: (d) copulating beetles Cantharis fusca; (e) brush-like coverage on the medial surface of abdonminal appendages in the male fly Dolichopus ungulatus used for attachment to the female; (f) specialized tarsal setae of the male beetle Leptinotarsa decemlineata. (g–i) Grooming, sampling, filtering (particle manipulation): (g) cleaning organ on the fore leg of the ant Formica polyctena; (h) ‘pseudotrachea’ of the labellum in the fly Calliphora vicina; (i) filter system of the spiracle in the tenebrionid beetle Tenebrio molitor.

In the course of evolution, males of different animal species have developed a number of specializations to attach to the female surface during copulation (figure 3e). In many animals, male and female are attached to one another during copulation with a kind of lock-and-key mechanism, which has not only mechanical but also sensory function and is also involved in the recognition of an interspecific mate. In the case of aquatic beetles, the specialized attachment system of males relies on the suction cup mechanism and is under a strong pressure of natural selection (Törne 1910; Aiken & Khan 1992; Miller 2003). There are some examples where such specialization is so strongly ‘overdeveloped’ (figure 3f) that males can even be hindered to perform normal walking on smooth surfaces (Pelletier & Smilowitz 1987). Operation of these systems is based on mechanical interlocking, friction (figure 3e), suction and/or capillary adhesion (figure 3f).

Many attachment devices are specialized for manipulation of particles. These structures are enormously diverse, and include systems used in grooming (cleaning), sampling of food particles and filtration. Grooming is a very important function for animals, especially when they live in dirty environments. This function is definitely related to adhesion because contamination is usually removed by specialized structures that attract both dust particles and dirt more strongly than other unspecialized surfaces. For example, many ants, wasps and bees (Schönitzer & Penner 1984; Schönitzer 1986; Schönitzer & Lawitzky 1987; Francouer & Loiselle 1988) bear flattened leg spines specialized for the cleaning of antennae (figure 3g).

Collecting pollen grains and food particles is also a function related to adhesion. Interestingly, such systems in the region of the mouthparts appeared independently in the evolution of such phylogenetically far-related animal groups as Insects, Crustaceans, Molluscs and even some Vertebrates (Arens 1989). In bees (Apoidea), systems responsible for collecting pollen grains are usually equipped with urticating bristles (Pasteels & Pasteels 1972; Hesse 1981). Some beetles bear setae similar to those found on tarsal adhesive pads (Krenn et al. 2005). In aquatic organisms, structures specialized for sampling are mostly based on adhesive secretions (tunicates, bivalve molluscs, some polychaetes). Sampling here is closely connected to the function of filtration and burrowing or tube building (some polychaetes and some insect larvae). Filtration systems are usually equipped with long bristles. Such systems are well known from mouthparts of aquatic invertebrates. The filtering system of insect spiracles is often composed of branched acanthae (figure 3i). Mouthparts (labellum) in flies (Brachycera, Diptera) bear so-called pseudotrachea (Gracham-Smith 1930; Dethier 1963; Zaytsev 1984; Elzinga & Broce 1986; Driscoll & Condon 1994), which are able to change the diameter of the filtration sieve, depending on the size of particles in the food (figure 3h).

3. Time scale of attachment

Different attachment systems in biology can be subdivided into three main functional groups depending on the time scale they are operating: permanent, temporary and transitory attachment (Tyler 1988; Flammang 1996; Scherge & Gorb 2001; figure 4). The first type of these systems is mainly observed in plants, sessile marine organisms, animal eggs, some insects' pupae, some parasitic animals, etc. For this purpose, different kinds of glues/cements or mechanical interlocking are used (figure 1). Temporary attachment is found in locomotory systems of lizards, tree frogs, some mammals, insects, spiders, cephalopods, echinoderms and other organisms. There are a number of systems responsible for temporary attachment of a pair of structures within the same organism. Additionally, temporary attachment can be subdivided into two subgroups, each with a different time scale of contact: short-term, where the attachment–detachment cycle lasts for maximally a few minutes (locomotory devices of geckos, flies, beetles and spiders), and long-term, where the cycle may last from minutes to days (cephalopods, echinoderms and arresting systems of the head and wings). The short-term systems mainly rely on van der Waals interactions, capillarity and viscous forces (Stefan adhesion), whereas the long-term ones usually use a suction-cup mechanism, mechanical interlocking, friction or glues (figure 1). Transitory attachment is observed in snails, sea anemones or some flatworms (turbellarians), which are able to move while adhering to the substrate (Denny & Gosline 1980; Denny 1981).

Classification of biological attachment devices according to the time scale of contact.

4. Mechanisms of attachment

(a) Mechanical interlocking: hook, snap, clamp and spacer

The hook is a widespread attachment principle across living nature (figures 1 and 2). It has mainly been reported from systems adapted for long-term attachment. Numerous specialized, hook-like structures are found in parasitic animals, adapted for attachment to particular surfaces of the host body (Kabata 1968; Kabata & Cousens 1972; Hunter & Rosario 1988). A huge variety of hook-like attachment devices has been described for parasitic mites. The most common example of the hook-like attachment device used for short-term attachment during locomotion is the tarsal claw, which is used to interlock with surface texture. Prolegs of butterfly caterpillars bear hooks, or crochets, surrounding the proleg sucker (Barbier 1985; Nielsen & Common 1991; Hasenfuss 1999). They are not homologous to imaginal claws, but serve a similar function. Fore and hind wings of bees, wasps, butterflies, aphids and other insects can be locked with each other with different systems of hooks (Nielsen & Common 1991).

The snap (lock-and-key) attachment principle includes systems with co-opted surface profiles: outgrowth and depression (figures 1 and 2). In addition, both surfaces can be covered with tiny cuticle protuberances or depressions. The snap principle is widely represented in organ structures related to copulation. For example, males of mayflies (McCafferty & Bloodgood 1989) and dragonflies attach to females with extremely specialized lock-and-key devices. The diversity of lock-and-key devices, even among insects, is huge. These snap systems are often supplemented with additional structures functioning according to hook, clamp and friction principles. The lock-and-key principle in the design of insect copulation apparatus, and its biological significance has been reviewed in detail elsewhere (Shapiro & Porter 1989). Snap-like mechanisms usually provide a long-term fixation and always need two precisely adapted structures. To attach two parts of the snap to each other, or to detach them, muscular force has to be applied.

The clamp principle is usually found in complex mechanical systems, which can attach to or hold onto diverse structures, or substrata, by the use of muscular force (figures 1 and 2). Usually, clamp arms have a particular curvature and are often supplemented by a variety of coverages. However, they are not necessarily adapted to one particular surface. Devices adapted for prey capture (in predators) or attachment to the host (in parasites) are often constructed as clamp-like structures. Biting mouth parts of insects can work as a clamp. Many species of crustaceans and insects use their chelae as an organ adapted for capturing, manipulating and processing prey. Chelae, and predatory legs in general, have convergently evolved in representatives of many groups of Arthropoda (Gorb 1995; Frantsevich 1998). Clamp-like mechanisms widely exist in functional systems adapted for copulation. The clamp segment usually flexes during capturing movement to the basal segment (Loxton & Nicholls 1979). The clamp often fits into the groove of the basal segment. The groove margin usually bears rows of spines, or setae, to enhance contact forces. In some crustacean chelae, the muscles belong to the slow type (Costello & Govind 1984). They contract slowly but produce a large force and can remain longer in the contracted condition.

Spacer-like attachment devices rarely occur in biological systems. Such systems are quite similar to the clamp-like mechanisms described above, because muscular force or hydraulics are involved in both principles. In the case of spacers, however, internal forces are used to transfer the system from the free condition into the working condition, and back. Usually, attachment itself does not require any muscular energy. In many cases, such systems are supplemented with hooked or rough surfaces to interlock with the supporting substratum, or to increase friction with its surface.

(b) Sucker

A sucker uses the difference between atmospheric pressure and the pressure under the suction cup (figures 1 and 2). Important properties of the cup are its concave shape, flexibility, smooth surface at the edge, and, in some cases, occurrence of muscles or muscular fibres that are responsible for the generation of low pressure. This principle widely exists in soft-bodied animals, such as worms and molluscs (Nachtigall 1974). However, numerous examples have also been found among insects and crustaceans. Suction cups are widely used by parasites and predators. Some species bear specialized suction cups or their arrays for attachment during copulation (Aiken & Khan 1992). True suckers are adapted for attachment to relatively smooth substrata (some stones, plant leaves and surfaces of other animals). In most functional systems, they provide long-term attachment. The sucker requires a muscle, deforming the cup, and specialized types of resilient tissues. Diverse fluids are used to provide better contact of the sucker margin to the substratum. This is essential for maintaining lower pressure under the cup.

(c) Frictional systems

Probabilistic fasteners are attachment devices composed of two surfaces covered with cuticular micro-outgrowths (figures 1 and 2). The attachment in such systems is based on the use of the surface profile and the mechanical properties of materials, the combination of which results in an increase of the contact forces in the contact zone. Attachment generated by these systems is fast, precise and reversible. Such systems have been described from head arresting devices (Gorb 1999b), unguitractor plate (Gorb 1996) and other intersegmental fixators of antennal and leg joints (Gorb 2004), ovipositor valvulae (Austin & Browning 1981; Gorb 2001) and wing attachment devices (Hammond 1989; Gorb 1998a, 1999a). The combination of morphological studies on biological systems, experimental data obtained in an artificial model system, and theoretical considerations based on a simple model of the behaviour of probabilistic fasteners with parabolic elements has demonstrated that the attachment force in this type of system is strongly dependent on the load force (Gorb & Popov 2002). At small loads, the load-to-attachment ratio is rather low, whereas a rapid increase of attachment was detected at higher loads. At very high loads, a saturation of the attachment force was revealed. A simple explanation of the attachment principle is that, with an increasing load, single outgrowths of both surfaces slide into gaps in the corresponding part. This results in an increase of lateral load acting on neighbouring elements. High lateral forces lead to an increase of friction between single sliding elements.

(d) Adhesion: glue, capillary effects and molecular forces

Materials and systems preventing the separation of two surfaces may be defined as adhesives. Why use adhesives for attaching materials to surfaces? There is a variety of natural attachment devices based entirely on contact mechanical principles (capillary interactions, viscous forces (Stefan adhesion) and van der Waals forces; Hanna & Barnes 1990; Autumn et al. 2002; Federle et al. 2002), whereas others additionally rely on the chemistry of polymers and colloids (diverse types of glues; Gorb 2001; Scherge & Gorb 2001; Habenicht 2002; figures 1 and 2). There are at least three reasons for using adhesives: (i) they join dissimilar materials, (ii) they show improved stress distribution in the joint, and (iii) they increase design flexibility (Waite 1983). These reasons are relevant to the evolution of natural attachment systems and man-made joining materials as well. Biological adhesion can be found at all levels of organization of living tissues. Cell contact phenomena have been extensively reviewed in the biomedical and biophysical literature.

The function of attachment using glues appeared very early in evolution; even unicellular organisms have a variety of cellular adaptations to adhesion to the substrate (Callow & Callow 2006). Many multicellular organisms often bear single secretory cells producing adhesive secretions (Flammang 1996). In many cases, however, cells are specialized into glands, which may be composed of several cell types. In the case of the glue, adhesive bond formation consists of two phases: contact formation and generation of intrinsic adhesion forces across the joint (Naldrett 1992, 1993). The action of the adhesive can be supported by mechanical interlock between irregularities of the contact surfaces. Increased surface roughness usually results in an increased strength of the adhesive joint. The last result, however, may be simply explained by increased contact area between contacting surfaces and the glue (Santos et al. 2005). Strong adhesion is also possible between two ideally smooth surfaces. If sufficient contact area at the interface between the substrate and the adhesive is reached, attractive forces act at the level of atoms and molecules of both contacting materials. van der Waals forces and hydrogen bonds are the most widespread interactions of this type. Electrostatic forces may also be involved: many biological glues are rich in polar and charged residues (amino acids or sugars) and interact with the substrate through electrostatic interactions (Waite 1983). Electrostatic charges seem to be involved in the adhesion of pollen grains to the surface of insect pollinators (Hesse 1981).

Adhesive organs, used for releasable attachment to substrates, as well as those involved in catching prey, demonstrate a huge diversity among living creatures. In their evolution, animals have developed two distinctly different mechanisms to attach themselves to a variety of substrates: with smooth pads or with setose/ hairy surfaces (Gorb & Beutel 2001). Owing to the flexibility of the material of the attachment structures, both mechanisms can maximize the possible contact with the substrate, regardless of its microsculpture. The tips of tenent setae are relatively soft structures (Niederegger et al. 2002). In flies, they are usually compressed, widened and bent at an angle of 60° to the hair shaft (Bauchhenss & Renner 1977). When walking on smooth surfaces, these hairs in flies and beetles produce secretion, which is essential for attachment (Ishii 1987). It has been previously hypothesized that capillary adhesion and intermolecular van der Waals forces may contribute to the resulting attachment force (Stork 1980; Dixon et al. 1990). The action of intermolecular forces is possible only at very close contact between surfaces. The forces increase when the contacting surfaces slide against each other. This may explain why flies sitting on a smooth undersurface always move their legs in a lateral–medial direction (Wigglesworth 1987; Niederegger & Gorb 2003). During these movements, pulvilli slide over the surface obtaining optimal contact. A contribution of intermolecular interaction to the overall adhesion has been previously shown in experiments on the adherence of beetles on a glass surface (Stork 1980). The presence of claws, decrease of air pressure, decrease of relative humidity or electrostatic forces do not influence beetle attachment on the smooth substrata. In the beetle Chrysolina polita (Chrysomelidae), the resulting attachment force directly depends on the number of single hairs contacting the surface. Recently, the contribution of intermolecular interaction and capillary force has been demonstrated for the fly Calliphora vicina in a nanoscale experiment with the use of atomic force microscopy (Langer et al. 2004). Smooth systems are composed of cuticles of unusual ultrastructure. The key properties of smooth attachment devices are deformability and the softness of the pad material having viscoelastic properties (Gorb et al. 2000; Jiao et al. 2000).

5. Biomimetic potential

The industry of adhesives is presently following three main goals (Hennemann 2000): (i) an increase in the reliability of glued contact, (ii) mimicking of natural, environment-friendly glues, and (iii) development of mechanisms for application of a minute amount of glue to the surface (figure 5). An additional challenge is the use of substances or mechanisms that allow multiple attachment and detachment, and enable attachment to the broadest variety of surfaces. Many biological attachment devices correspond to some of these requirements. One such example is the hairy surface of the leg pulvillus in flies. This system uses a secretion enabling hairs to attach and detach to diverse substrata very quickly. The hair design includes a mechanism that delivers the secretion, in extremely small amounts, directly to the contact area, and only then when the contact to the substrate is achieved (Gorb 1998b).

Hairy and smooth leg attachment pads are promising candidates for biomimetics of the robot soles adapted for locomotion. Similar principles can be applied to the design of microgripper mechanisms with an ability to adapt to a variety of surface profiles. Some hairy systems (see paper on gecko adhesion by Autumn & Gravish 2008) seem to operate with dry adhesion and do not require supplementary fluids in the contact area. Contacting surfaces in such devices are subdivided into patterns of micro- or nanostructures with a high aspect ratio (setae, hairs and pins). The size of single points gets smaller and their density higher as the body mass increases (Scherge & Gorb 2001). We have explained this general trend by applying the JKR contact theory (Johnson et al. 1971), according to which splitting up the contact into finer subcontacts increases adhesion (Autumn et al. 2002; Arzt et al. 2003).

During the last few years, several groups worldwide have made good progress in developing biologically inspired patterned adhesives, which mimic the geometry of biological systems at different scales (Geim et al. 2003; Glassmaker et al. 2004; Hui et al. 2004; Peressadko & Gorb 2004; Crosby et al. 2005; Northen & Turner 2005; Autumn et al. 2007). Patterned surfaces usually have significantly higher adhesion on a smooth surface than a smooth sample made out of the same material. This effect is even more pronounced on curved substrata (Peressadko & Gorb 2004). An additional advantage of patterned surfaces is the reliability of contact on various surface profiles and the increased tolerance to defects of individual contacts. The best performance (adhesive strength of 60–100 KPa) has been obtained for the patterns with mushroom-like contact elements (Daltorio et al. 2005; Kim & Sitti 2006; Gorb et al. 2007). The foil with the mushroom-like microstructure pattern demonstrates considerably higher pull-off force per unit apparent contact area and is less sensitive to contamination by dust particles (Gorb et al. 2007). After being contaminated and washed with soapy water, the initial adhesive properties can be completely recovered. These properties, deviating strongly from those measured in the flat control samples made of the same polymer, can be explained by the difference in the contact behaviour of single contact elements.

Walking machines usually use suckers to hold onto vertical surfaces and under a surface. A primary disadvantage of this attachment principle is large energy consumption for vacuum maintenance. The novel biologically inspired materials, as mentioned above, may enable future robots to walk on smooth surfaces regardless of the direction of gravity. Mini-Whegs, a small robot (120 g) that uses four-wheel legs for locomotion, was recently converted to a wall-walking robot with compliant, adhesive feet (Daltorio et al. 2005). The robot is capable of ascending vertical smooth glass surfaces using the novel adhesive. The foot material maintains its properties for nearly twice as many walking cycles before becoming contaminated. Similar efforts to apply knowledge from biological adhesive systems to robotics have been undertaken by Stanford's robotic group. In the Stickybot robot, which is able to walk on the wall using structured adhesive polymers, a gecko-like peeling principle of the foot detachment is excellently applied (Kim et al. 2007).

There are a lot of activities worldwide in developing surfaces with enhanced or controllable friction. Such biomimetic products as Velcro fasteners or 3M Dual-Lock or Mate-Lock are well known. Recently, the company Continental has developed a winter tyre with honeycomb profiles similar to those existing on the attachment pads of the grasshopper Tettigonia viridissima (Gorb et al. 2000) and tree frogs (Hanna & Barnes 1990). The company promises enhanced wear performance on the dry road, less aquaplaning and better braking on wet roads, substantially improved lateral guidance, better grip and more traction on ice.

We believe that the huge diversity of biological attachment mechanisms will continuously inspire material scientists and engineers to develop new materials and systems. That is why broad functional comparative studies on biological systems has to be intensified, in order to extract essential structural, chemical and mechanical principles behind their functions. Using living nature as an endless source of inspiration might be another reason for saving biological diversity on Earth.

Acknowledgments

This study was supported by the Federal Ministry of Education, Science and Technology, Germany (project InspiRat 01RI0633D).

Footnotes

One contribution of 7 to a Theme Issue ‘Nanotribology, nanomechanics and applications to nanotechnology II’.

Wing-folding mechanism of beetles, with special reference to investigations of adephagan phylogeny (Coleoptera). In Carabid beetles: their evolution, natural history, and classificationErvinT, BallG.E, WhiteheadD.R1989pp. 113–180. Eds. The Hague, The Netherlands:Junk Publishers.